Today there exist measuring devices working by the principle of liquid displacement, such as for use when measuring e.g. biogas flows, e.g. in the anaerobic biodegradation process. In the anaerobic biodegradation process, gas flow rate and methane content are key parameters for evaluating the process performance. In laboratory scale experiments, the bioreactor volume is usually quite small, often only a few liters. The biogas production is also low, especially during the startup period, often a few milliliters per hour. Measuring such a low flow rate of biogas precisely is therefore not an easy task. The commercially available gas flow meters are usually not suitable for measuring such a small amount of gas with low flow rates. To solve this practical problem, several gas-measuring systems have been devised. Most of these systems are based on the principle of liquid displacement. The volume of a gas sample immersed in a fluid is determined by measuring the volume of the displaced fluid. Although there are gas flow meters being reported and/or commercialized based on the liquid displacement technique and pressure analysis technique, the design and construction of these flow meters are still too complicated. As a result, cost for these meters is too high which makes it difficult to implement widely.
One example of such a known device is the measuring device described in U.S. Pat. No. 4,064,750, which discloses an inverted bucket having a pair of gas entrapping compartments alternatively positioned over the gas discharging end of an inlet conduit submerged within a body of liquid. The bucket is pivotally displaced between its operative positions by the buoyant force exerted by a preset quantity of gas accumulated. The gas is accumulated in one compartment while the gas previously accumulated in the other compartment escapes. Displacement of the bucket is detected and registered by a counter mechanism.
One of the drawbacks with the device disclosed in U.S. Pat. No. 4,064,750 is that the device disclosed therein comprises quite complicated and costly components. Moreover, the counter system used for measuring the gas flow is a very old and complicated system, usually a mechanical counter system. The device disclosed in U.S. Pat. No. 4,064,750 is very similar to a device disclosed in patent DE 4006508, at least in terms of functionality and working principle. Also this device is complicated and comprises costly components.
Yet a further measuring device is disclosed in U.S. Pat. No. 5,325,725, which discloses a measuring cup, mounted upside down on a pivot. The device has means for guiding gas to a position under the cup, so that gas accumulates in the cup. When sufficient gas is gathered under the cup, the combined buoyancy of the cup and gas will cause the cup to rotate whereby a predetermined amount of gas, all or only a portion of the gas, will be released from the cup to an outlet.
A drawback with the device disclosed in U.S. Pat. No. 5,325,725 is that the device does not provide enough accuracy. Firstly, the design of the measuring cup is such that it does not respond to a sufficiently small amount of excess gas. Secondly, simultaneous release of all gas is not specifically promoted since the design is such that partial release of gas is just as satisfactory, as regards the use of the device.
Another type of measuring device is disclosed in U.S. Pat. No. 5,092,181. U.S. Pat. No. 5,092,181 discloses a method and apparatus for measuring minute gas flows and/or accumulations wherein a gas flow is bubbled through a defined path in separated form and a sensor is used to detect and count each bubble. The bubble count signal can be used to calculate gas accumulation, or to calculate gas flow rate if made during a defined time period, or to meter the flow of the gas to a point of utilization.
There are some drawbacks with the device according to U.S. Pat. No. 5,092,181. As understood from above, the device is an apparatus of optical bubble counter system. One probable problem with such systems is their inaccuracy. When just counting bubbles and then measuring the gas flow, the assumption is made that the volumes of the different bubbles are the same, which is not correct. This assumption works alright when the gas flow is low and constant. However, the size of the bubbles may vary with the flow velocity, and hence, this assumption should render an inaccurate flow measurement for most flows as these varies. According to U.S. Pat. No. 5,092,181, the volumes of the different bubbles are said to be substantially equal, but this may thus in fact be an error source.
One of the objects of the present invention is to provide a gas measuring device working by the principles of liquid displacement, which is effective, gives an accurate flow measurement, and is inexpensive to produce, both in terms of material and production costs. Other objects of the present invention will be apparent from the summary of the invention and the detailed description below.